The goal of this application is to define the factors that have a positive or negative effect on the risk of transmission of HIV-1 through sexual activity in gay men. To address this question, we will conduct a non-concurrent prospective study utilizing specimens obtained from two groups of gay couples. The first group remained discordant for HIV, i.e., one member of the couple is HIV-1-seropositive (HIV+) and the other is seronegative (HIV), despite high-risk sexual activity. The second group of couples were initially discordant, but then HIV infection was transmitted from the HIV+ to the HIV- partner. Our preliminary data show that the discordant couples maintained their discordant status over several years in which they engaged in high-risk sex, and this status was confirmed by all available diagnostic tests (HIV culture, serology, and polymerase chain reaction (PCR)). In this proposal we will attempt to delineate specific factors that could explain the lack of transmission of HIV in these couples, including characteristics of the HIV+ partner (HIV properties or host immunological factors) or the HIV- partner (host immunological or susceptibility factors). Using a repository of banked specimens of PBMC, semen, serum, and HIV isolates collected at times when high-risk behaviors were occurring in both sets of couples, we will test the hypotheses that lack of transmission of HIV is due to 1) low viral load in semen or peripheral blood, and/or reduced viral infectiousness in the non-transmitting HIV+ partners; 2) strong specific or non-specific anti-HIV immune responses (neutralizing antibody, activated CD8+ T lymphocytes, activated natural killer cells) in the HIV+ partner; 3) genetically resistant CD4+ lymphocytes in the HIV- partner; or 4) strong specific or non-specific anti-HIV immune responses (activated CD8+ T lymphocytes, activated natural killer cells) in the HIV- partner. These hypotheses will be tested by comparing the measurements obtained in the nontransmitting couples to those in the transmitting couples. In these studies, special advantage will be taken of the ability to test the HIV- partner's immune responsiveness against a specific HIV isolate to which he has been exposed, namely that of the HIV+ partner. Whether the HIV- men have immunological memory for HIV antigens will also be tested, as evidence that they have been exposed and may have a protective immunologic response, by measuring cytokine production. The data obtained should clarify the pathogenesis of HIV infection and the relative importance of factors that contribute to the risk of transmission of HIV.